Bread-Bakers v104.n008.7

Here is a copy of the recipe for English Muffin Bread I baked using a 
Breadman Ultimate machine. The texture and taste were fine , however the 
center of the bread caved in. I wonder what I might have done wrong to 
cause this.

1 1/2 cups warm water
3     tablespoons sugar
1 1/2 teaspoon salt
3 1/2 cups bread flower
1/4 cup non fat dry milk
1/4 teaspoon baking soda
2 1/4 teaspoons yeast

Process on basic white cycle: use medium/normal crust.
Do not use delayed-bake feature.